Snubbing is used in the oil and gas industry for performing well interventions/well completions on wells that are under pressure. A snubbing unit is used to force jointed tubing into and out of a well while maintaining the pressure in the well (i.e. not requiring the well to be killed). Unlike wireline or coiled tubing, which have an outer diameter that remains constant throughout its length, jointed tubing has a varying outer diameter (i.e. the collars where the sections of jointed tubing are connected tend to have a larger outer diameter) requiring the collars to be taken into account when the jointed tubing is forced into or out of a well.
A snubbing unit is typically a relatively tall structure. It must be tall enough to lift a section of jointed tubing above the well head to be connected to a section of jointed tubing extending out of the well head. It typically also has some hydraulically powered components. A stationary slip and a hydraulically powered traveling slip are also typically provided to force the jointed tubing in and out of the well. The traveling slips are used to grab the pipe section and drive the tubing section into or out of the well. The stationary slips are used to hold the tubing section in place in the well, while the stationary slips release the tubing section and are repositioned for the next stroke.
Snubbing units also typically require pressure control components to maintain the pressure in the well while the tubulars are being snubbed into or out of the well. These components provide sealing to the outside of the tubulars while the jointed tubing is being forcibly inserted into the well head and have to accommodate the increased outside diameters at the joints of the tubulars.
Typically, snubbing is often done with standalone structures. The standalone structure must be installed at the well head and the snubbing of the tubular string performed. Once the snubbing has been completed, the stand alone snubbing unit can be removed and a service rig brought in to perform completion services on a well. If for any reason more snubbing operations have to be performed before the completion services are finished, the service rig must be removed from the well head and the standalone structure put back in place around the well head to perform the additional snubbing operations.